Default settings of TCP keep-alive are not always feasible,
take Linux for example, the duration a connection needs to be
idle before sending out the first keep-alive probe is two hours,
which makes detecting dead connections extremely deferred. Thus,
enabling TCP keep-alive will be to little avail.
This sets TCP_KEEPIDLE, TCP_KEEPINTVL, and TCP_KEEPCNT when enabling TCP
keep-alive mechanism to make it practicable.
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